1. Field of the Invention
The present invention relates to an eyepiece which is used, for example, in binoculars or in a microscope, and more specifically relates to an ocular lens including four lens elements.
2. Description of Related Art
An eyepiece for a microscope comprised of four lens elements is disclosed, for example, in Japanese Unexamined Patent Publication No. 60-57315. The eyepiece disclosed in JPP '315 is made of a glass having a high refractive index.
An eyepiece for binoculars, made of a plastic material less expensive than glass, is also disclosed, for example, in Japanese Unexamined Patent Publication No. 3-87710 or No. 3-87709. The eyepiece in these publications is comprised of two positive and one negative lens elements, and two positive and two negative lens elements, respectively.
In order to provide a plastic lens with a necessary power, it is necessary to reduce the radius of curvature thereof. Since the selection of a refractive index for a plastic lens is restricted in comparison with a glass lens, if the radius of curvature of a lens surface is small, light (peripheral light) passing though the peripheral portion of the lens has a large comatic aberration, or a total reflection of the peripheral light at the lens surface occurs. To prevent this, in the publications mentioned above, the plastic eyepiece is comprised of positive and negative lens elements which are cemented to each other. The surfaces, of these lens elements, having the smallest radius of curvature are cemented.